The present invention relates to a charging and discharging circuit for a pulse laser for use in an excimer laser or other gas laser unit.
Conventionally, there is known a pulse laser for oscillating a laser beam in pulses by discharging at a predetermined frequency between discharge electrodes to excite laser medium such as excimer laser.
Referring to FIG. 6, there is shown a block diagram of a charging and discharging circuit for a pulse laser related to a prior art.
In FIG. 6, the charging and discharging circuit comprises a first capacitor C0, a high-voltage power supply 18 for charging the first capacitor C0 with high-voltage electric charges, and a discharging circuit 3 for applying these electric charges to a portion between discharge electrodes 5, 5. The electric charges applied to the portion between the discharge electrodes 5, 5 discharge between the discharge electrodes 5, 5 and excites a laser medium to oscillate a pulse laser beam 11. The oscillated pulse laser beam 11 reaches a processing machine 15 so as to be processed.
The high-voltage power supply 18 and the discharging circuit 3 are connected to a laser controller 4 for controlling laser oscillation. The high-voltage power supply 18 starts charging the first capacitor C0 together with an input of a charging command signal H1 from the laser controller 4. The discharging circuit 3 applies the electric charges which have been applied to the first capacitor C0 to the discharge electrodes 5, 5 together with an input of a trigger signal Tr from the processing machine 15.
If this type of a pulse laser is used for precision processing such as, for example, laser lithography, energy of the pulse laser beam 11 per pulse (hereinafter, pulse light emission amount) must be controlled precisely. For the precise control, an interpolar voltage VC applied across the first capacitor C0 need be controlled for each pulse.
In other words, the processing machine 15 measures the pulse light emission amount by using a light emitting monitor 12 after oscillating the pulse laser beam 11 and requests a pulse light emission amount for the next oscillation of the laser controller 4. This pulse light emission amount is called pulse light emission request amount Px and transmitted to the laser controller 4 by means of a pulse light emission amount request signal P. The laser controller 4 performs a predetermined arithmetic operation on the basis of this pulse light emission request amount Px to calculate a final target value VF of the interpolar voltage VC and transmits the value to the high-voltage power supply 18 by means of a target value signal VH.
Referring to FIG. 7, there is shown a timing chart of charging and discharging according to the prior art.
In FIG. 7, electric charges applied across the first capacitor C0 together with the input of the trigger signal Tr at time t1 are applied to a portion between the charging electrodes 5, 5, by which the pulse laser beam 11 is oscillated.
The laser controller 4 receives the pulse light emission request amount Px from the light emitting monitor 12 at time t2 after a completion of oscillating the pulse laser beam 11. On the basis of the pulse light emission request amount Px, the laser controller 4 performs an arithmetic operation for calculating the final target value VF of the interpolar voltage VC for charging the first capacitor C0 at the next pulse laser oscillation between the time t2 to time t3.
Subsequently at the time t3, the laser controller 4 transmits the calculated final target value VF by means of a target value signal VH to the high-voltage power supply and inputs a charging command signal H1 to the high-voltage power supply 18, by which the high-voltage power supply 18 starts charging the first capacitor C0. When the interpolar voltage VC reaches the final target value VF at time t4, the high-voltage power supply 18 terminates the charging and applies the electric charges to the portion between the electrodes 5, 5 together with an input of the trigger signal Tr at time t5. It causes discharging, by which the pulse laser beam 11 is oscillated.
The above prior art, however, has a problem as described below.
In other words, if the pulse light emission amount of the pulse laser beam 11 is the same, a processing amount per unit time in laser processing with the processing machine 15 is almost proportional to an oscillation frequency of the pulse laser. In order to increase the pulse light emission amount, the pulse laser needs to be large-sized and it leads to an increase of a device cost, and therefore it is required to increase the oscillation frequency of the pulse laser to perform a further large amount of processing in the same pulse laser. It is particularly needed for an excimer laser and a vacuum ultraviolet laser used as light sources for laser lithography for producing semiconductors.
According to the prior art, a time required from the pulse laser oscillation to the next oscillation mostly accounts for:
(1) time ts from the time t2 when the laser controller 4 starts the arithmetic operation of the final target value VF to the time t3 when the laser controller 4 terminates the arithmetic operation to specify the final target value VF for the high-voltage power supply 18 and
(2) time tp from the time t3 when the charging is started to the time t4 when charging the first capacitor C0 is terminated.
In other words, to increase the oscillation frequency, the time ts and the time tp need to be reduced.
To reduce the time ts, however, an ability of performing the arithmetic operation of the laser controller 4 should be increased, and therefore an arithmetic unit such as an expensive computer is required. In addition, to reduce the time tp, a charging ability of the high-voltage power supply 18 should be increased, and therefore a large-sized and more expensive high-voltage power supply 18 is needed.
As described above, the prior art has a problem that an operation from discharging to a completion of the next charging takes a long time, so that it is hard to increase the oscillation frequency of the pulse laser beam 11.
The present invention is provided from a viewpoint of the above problem. It is an object of the present invention to provide a charging and discharging circuit for a pulse laser which enables the pulse laser to increase an oscillation frequency.
In order to achieve the above object, there is provided a charging and discharging circuit for a pulse laser according to a first aspect of the invention, comprising a high-voltage power supply for charging a capacitor with electric charges until an interpolar voltage of the capacitor reaches a final target value, a discharging circuit for discharging the electric charges in pulses between discharge electrodes to excite a laser medium and oscillating a pulse laser beam, a pulse monitor for detecting a pulse light emission amount per pulse of the pulse laser beam, and a laser controller for calculating a final target value of an interpolar voltage for the next charging of the capacitor on the basis of the pulse light emission amount after the laser oscillation and outputting the value to the high-voltage power supply, wherein the high-voltage power supply starts charging the capacitor with electric charges toward a predetermined primary target value before the laser controller calculates the final target value of the interpolar voltage of the capacitor and wherein the charging is performed up to the calculated final target value after the final target value is calculated.
According to the first aspect of the invention, the high-voltage power supply starts charging the capacitor between the poles thereof before the laser controller calculates the final target value on the basis of the pulse light emission amount of the pulse laser beam. Accordingly, the charging is continued during a time period in which the laser controller calculates the target value of the interpolar voltage, which reduces a time between the pulse laser beam oscillation and the completion of charging the capacitor, by which an oscillation frequency of the pulse laser beam can be increased.
According to a second aspect of the invention based on the first aspect of the invention, the high-voltage power supply starts primary charging of the capacitor with electric charges toward the primary target value, keeps the value which has reached the primary target value, and then performs final charging of the capacitor up to the final target value.
This ensures the same action and effect as for the first aspect of the invention.
According to a third aspect of the invention based on the first aspect of the invention, the high-voltage power supply starts primary charging of the capacitor with electric charges toward the primary target value and then performs final charging of the capacitor up to the final target value so that a charging amount per unit time always keeps a constant level.
This ensures the most efficient use of a charging ability of the high-voltage power supply in addition to the same action and effect as for the first aspect of the invention.
According to a fourth aspect of the invention based on the first aspect of the invention, the high-voltage power supply rapidly performs primary charging of the capacitor with electric charges toward the primary target value and after reaching the primary target value, gradually performs final charging of the capacitor up to the final target value.
This ensures a reduction of a time for the primary charging and an accuracy of reaching the final target value in the final charging in addition to the same action and effect as for the first aspect of the invention.